Pipe Welding Defects
Many of the defects that occur in pipe welding have, of necessity, already been discussed in previous chapters. In this chapter additional information will be given on the causes of these defects and preventive measures required to avoid their occurrence. The subject will be covered from the point of view of the welder, rather than that of the engineer.
Arc Strike Cracking
There are many reasons why cracking occurs in the weld and in the adjacent weld-affected zone. One cause of cracking is carelessly striking the arc outside of the weld groove, or in the area in which the weld deposit is to be made, Fig. 13-1.
When the arc is struck, the metal with which it is in contact is very rapidly heated. Then when the arc is moved on it is very rapidly cooled again; in effect, it is quenched. This affects only a relatively small volume of metal, the remainder not being heated appreciably.
As explained in Chapter 11, when the carbon content in steel is high enough, heating it to above the upper critical temperature and quenching it results in the formation of martensite. Low-carbon martensite can even form in low-carbon steel if the quench is drastic enough. With the addition of alloying elements, martensite will form at a lower carbon content and at a slower cooling rate. The martensite forms while the metal is cooling at some temperature below 700F and metal that has formed into martensite expands. This expansion of the metal results in internal stresses that may cause small micro-cracks to occur on, or just below, the surface.
If the arc is struck in the area over which the bead is to be deposited, the metal that is stressed or possibly cracked is melted a short time later as the bead is deposited, which eliminates the problem. On the other hand, if the arc is struck outside of the weld zone, the stresses and the cracks remain.
If cracks resulting from the arc strike appear on the surface, a weld bead must be deposited over them. The bead should be made reasonably long and wide in order to increase the amount of heat input to the defective area, thereby reducing the cooling rate. Inter-
Fig. 13-і, Deposits caused by striking the arc outside of the weld joint.
nal stresses can be corrected by postheating; i. e., by heating the affected area with an oxyacetylene torch allowing the metal to cool slowly. A most effective way to prevent the occurrence of these stresses and cracks is to preheat the weld-affected area so that the cooling rate is slowed down. Of course, the cardinal rule to follow is never to strike the arc outside of the weld joint.